1. Field of the Invention
This invention relates generally to systems for locating and tracing buried objects and more particularly to a walk-over locator apparatus having improved manufacturability and usability.
2. Description of the Related Art
There are many situations where is it desirable to locate buried utilities such as pipes and cables. For example, before starting any new construction that involves excavation, worker safety and project economic concerns require the location and identification of existing underground utilities such as underground power lines, gas lines, phone lines, fiber optic cable conduits, CATV cables, sprinkler control wiring, water pipes, sewer pipes, etc., collectively and individually herein denominated “buried objects.” As used herein, the term “buried objects” includes objects located inside walls, between floors in multi-story buildings or cast into concrete slabs, for example, as well as objects disposed below the surface of the ground. If excavation equipment such as a backhoe hits a high voltage line or a gas line, serious injury and property damage may result. Unintended severing of water mains and sewer lines generally leads to messy and expensive cleanup efforts. The unintended destruction of power and data cables may seriously disrupt the comfort and convenience of residents and bring huge financial costs to business.
When locating buried objects before excavation, it is further desirable to determine the approximate depth of the objects. This is generally attempted by measuring the characterizing emission field strength at two locations and analyzing the differences to infer the location of the emission source. However, there are many instances where the land that is to be excavated may be traversed or crisscrossed by several different utilities such as an electrical power cable, a water line, a gas line, a sewer pipe and a communications line. It is highly desirable to be able to determine their paths and their depths all at the same time. Also, many sites are host to a variety of overhead power and related lines, which emit electromagnetic fields that cannot be readily segregated from the emissions of similar buried lines. Some transmitters known in the art can produce several different signals at different frequencies for application to the same underground object or even to different underground objects, but a problem with these systems arises when several pipes are located in the same area and the location of all pipes is desired. Signals transmitted by several pipes can interfere and complicate the detection process.
Over the years, practitioners in the art have proposed numerous refinements to the magnetic field detector intended to facilitate the location of underground objects. For example, Mercer [John E. Mercer, “History of Walkover Locating Technology,” Int'l Constr. & Utility Equip. Expo, Louisville, Ky., 23-25 Sep. 1997] discusses a number for significant improvements dating from as early as 1933. Similarly, Roberts [Roy T. Roberts, “The History of Metal Detectors,” Western & Eastern Treasures Magazine, September 1999] describes the locator art from Michael Faraday (1831) through George Hopkins (1881), including the Hopkins ore-finding device employing an orthogonal sensor array (1904) to Harry Fore (1946). More recent proposals include, for example, U.S. Pat. No. 6,005,532 issued to Ng on Dec. 21, 1999, which discloses an orthogonal antenna arrangement and method that may use two or three identical antenna members to form, for example, a three axis orthogonal antenna assembly may be formed by receiving the two orthogonal axis antenna subassembly in the predetermined configuration of the through hole of a third one of the antenna members in a way which positions the axis of the antenna pattern defined by the third antenna member orthogonally with respect to the axes of the antenna patterns defined by the first and second antenna members. Ng's nested printed-circuit board (PCB) supported coil structures appear to be relatively bulky and of too few turns for useful sensitivity in the lower frequency regions. Although Ng's design permits the coils to be made nearly identical, the intersected regions in each coil are not symmetric, which introduces an orientation-specific non-uniform response to the field sensitivity over the operating frequency region. Also, U.S. Pat. No. 5,640,092 issued to Motazed et al. on Jun. 17, 1997, shows a sensor (FIG. 5) comprising a ferrite ball wound with three orthogonal coils. The device disclosed by Motazed et al. appears to be relatively heavy and costly to fabricate and may exhibit a number of complex circuit resonances at high frequencies and a relatively narrow operating frequency region. Ferrite cores also exhibit disadvantageous sensitivity to changes in operating temperature because ferrite permeability varies markedly with operating temperature.
The above-incorporated commonly-assigned patent applications also propose several improvements to the magnetic field measurement and line locating art, including the use of simultaneous measurement of magnetic field vectors in a plurality of independent frequency regions and the introduction of multiple 3D sensor arrays for measuring magnetic field vectors and the introduction of an improved graphical user interface (GUI) for line tracing. But there remains a clearly-felt need in the portable locator art for improved manufacturability. Modern sensor techniques require more sensor precision and sensitivity without concomitant increases in manufacturing costs, for example. Similarly, modern utility line tracing places complex demands on the locator user, who may be obliged to detect one or more buried objects in a crowded or electromagnetically noisy environment. The above-incorporated commonly-assigned patent applications also propose several improvements to assist the user in recursively processing a large amount of information to determine an object location. But there remains a clearly-felt need in the portable locator art for improved usability through more intuitive user interface (UI) designs, for example.
As described herein, the portable locator is generally small enough to be conveniently human-portable and battery-powered. As used herein, the term “battery” generally denominates a direct-current (DC) voltage source containing cells that convert chemical energy to electrical energy, but includes without limitation the usual alkaline, lithium-polymer and nickel-cadmium (NiCad) electrolytic cells, for example, as well as the more exotic power sources such as the solar photovoltaic cell, the hydrogen fuel cell and the methanol fuel cell, and the super capacitor and flywheel energy stores, for example.
Other practitioners have also proposed usability improvements. For example, U.S. Pat. No. 6,819,109 issued to Sowers et al. on Nov. 16, 2004 and assigned to Schonstedt Instrument Company, discloses a wand-type magnetic metal detector having a telescoping wand that may be shortened sufficiently to permit the detector to be comfortably carried in a holster fastened on a user's body, thereby freeing the user's hands for tasks other than carrying the detector.
Locator usability is important for effective location of buried objects. Effective detection and tracing of utility lines is vital to the safety of field personnel for many reasons; for example, the unplanned rupture of a high-pressure natural gas line can endanger the lives of everybody in the vicinity. Such a system must provide for the simultaneous detection and identification of either a passively-emitting buried object such as a ferromagnetic mass or an energized power cable or an actively-energized buried object such as a conductive pipe energized by means of an external transmitter signal or a non-conductive conduit occupied by an energized sonde, or all simultaneously, for example.
Another well-known problem with portable locator usability well-known in the art is the problem of the “quality” of a location indication. Low signal strength, nearby ferromagnetic objects and interfering EM fields are known to distort the EM field used by the locator to detect and locate the buried object sought by the user, who is then obliged to tease out the causes of signal fluctuations and ambiguities by experimenting with the operation of the portable locator. This usability problem may result in false positives and other errors (e.g., errors in depth measurement) in locating underground utility lines, for example. Such errors may result in unnecessary loss of user efficiency and safety.
Although practitioners in the art have long known about the usefulness of a pair of side-by-side sensing coils for measuring the local magnetic field gradient, (e.g., see U.S. Patent Nos. 4,387,340, 4,520,317, 5,043,666 and D475,936), until now, the combination of a redundant horizontal magnetic gradiometer with a pair of 3D B-field sensor arrays was unknown in the art. Moreover, the utility of such a combination is apparently counterintuitive in the art because (a) the improved usability of measuring the horizontal B-field gradient separately from the two displaced 3D B-field vectors was unknown in the art until now; (b) the additional sensor channels necessary for adding a separate horizontal gradiometer significantly increase locator system cost with no apparent performance advantages; and (c) locating additional sensor coils near the existing locator sensor arrays introduces mutual inductance couplings that are known to reduce the accuracy of the locator system.
These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.